Radio Frequency Interference (RFI) can be described as any unwanted radio frequency signal, which interferes with another, desired signal. This can show up as an annoying popping or crackle on a loud-speaker, unwanted pulses, marks on a sounder paper, or spots on a radar screen.
There are two ways for the noises to enter a device via the antenna. They can be conducted along the power leads and other wiring into the device, or they can be radiated from the noise source and its wiring into the device's antenna system, and then into the device.
Filtering the power wiring into the device can eliminate conducted interference. Most equipment has adequate filtering built in to eliminate conducted interference, but further or specific tuning of an antenna is difficult and follows no set pattern of instruction given the variable sources.
Radiated interference is the most serious problem normally encountered. Many device antenna systems and receivers have been designed for reception of very weak signals. Unfortunately the receiver cannot discriminate between these weak signals and random noise if both are on the same frequency, and if the noise has comparable strength to the required signal, then the signal is completely buried in the noise.
Radiated noise is caused by any abrupt change in current flowing through a conductor. Current flowing creates a magnetic field around the conductor. When this current is changed this causes a change in the magnetic field. The changing magnetic field travels as an electro-magnetic wave for some distance from the conductor, and it is this traveling field which is radiated to the antenna.
There are three basic methods of reducing RFI. The first is to prevent the device interference from reaching the antenna by shielding. If the noise source is totally enclosed in a metal can, then the noise is contained and cannot reach the antenna.
The second and more practical method of noise suppression is to eliminate the noise at the source. This is achieved by installing filtering or smoothing circuits across the noise producing device or contacts. A capacitor across a contact slows down the rapid change in current when the contact is opened. The slowed change in current through the conductors results in a weaker magnetic field around the conductor and therefore less radiated interference. The slowed change in current through the conductors results in a weaker magnetic field around the conductor and therefore less radiated interference.
The third important part of noise suppression is bonding. Bonding or grounding should provide a low resistance path to ground for any device frequency noise which is present. This path ensures that the noise is conducted away to ground instead of being radiated. Moreover, there always is interest either by the DoD or by commercial entities to change the beam shape of the antenna to better transmit to the receiver to change the emission frequency to accommodate the mission or application. In both cases, this tuning can be achieved using the same procedure as described in this invention.